This invention relates to wireless communication and, more particularly, to techniques for effective wireless communication in the presence of fading and other degradations.
Recently, some interesting approaches for transmitter diversity have been suggested. A delay diversity scheme was proposed by A. Wittneben in xe2x80x9cBase Station Modulation Diversity for Digital SIMULCAST,xe2x80x9d Proceeding of the 1991 IEEE Vehicular Technology Conference (VTC 41st), PP. 848-853, May 1991, and in xe2x80x9cA New Bandwidth Efficient Transmit Antenna Modulation Diversity Scheme For Linear Digital Modulation,xe2x80x9d in Proceeding of the 1993 IEEE International Conference on Communications (IICC ""93), PP. 1630-1634, May 1993. The proposal is for a base station to transmit a sequence of symbols through one antenna, and the same sequence of symbolsxe2x80x94but delayedxe2x80x94through another antenna.
U.S. Pat. No. 5,479,448, issued to Nambirajan Seshadri on Dec. 26, 1995, discloses a similar arrangement where a sequence of codes is transmitted through two antennas. The sequence of codes is routed through a cycling switch that directs each code to the various antennas, in succession. Since copies of the same symbol are transmitted through multiple antennas at different times, both space and time diversity are achieved. A maximum likelihood sequence estimator (MLSE) or a minimum mean squared error (MMSE) equalizer is then used to resolve multipath distortion and provide diversity gain. See also N. Seshadri, J. H. Winters, xe2x80x9cTwo Signaling Schemes for Improving the Error Performance of FDD Transmission Systems Using Transmitter Antenna Diversity,xe2x80x9d Proceeding of the 1993 IEEE Vehicular Technology Conference (VTC 43rd), pp. 508-511, May 1993; and J. H. Winters, xe2x80x9cThe Diversity Gain of Transmit Diversity in Wireless Systems with Rayleigh Fading,xe2x80x9d Proceeding of the 1994 ICC/SUPERCOMM, New Orleans, Vol. 2, PP. 1121-1125, May 1994.
Still another interesting approach is disclosed by Tarokh, Seshadri, Calderbank and Naguib in U.S. application, Ser. No. 08/847635, filed Apr. 25, 1997 (based on a provisional application filed Nov. 7, 1996), where symbols are encoded according to the antennas through which they are simultaneously transmitted, and are decoded using a maximum likelihood decoder. More specifically, the process at the transmitter handles the information in blocks of M1 bits, where M1 is a multiple of M2, i.e., M1=k*M2. It converts each successive group of M2 bits into information symbols (generating thereby k information symbols), encodes each sequence of k information symbols into n channel codes, and applies each code of a group of codes to a different antenna.
When knowledge of the channel is available neither at the transmitter nor at the receiver, the above schemes require the transmission of pilot symbols. For one transmit antenna, differential detection schemes exist that neither require the knowledge of the channel nor employ pilot symbol transmission. These differential decoding schemes are used, for instance, in the IEEE IS-54 standard. This motivates the generalization of differential detection schemes for the case of multiple transmit antennas.
A partial solution to this problem was proposed in U.S. patent application Ser. No. 09/074,224 filed on May 7, 1998, where the detected sequence is used to estimate the channel at the receiver, and those estimates are fed back and used to detect the next transmitted set of symbols. Therefore, the ""224 patent application disclosure can be thought of as a joint channel and data estimation.
Improvement in the art is realized by utilizing the fact that a space time encoding at the transmitter can be constructed where the symbols transmitted over a plurality of antennas in the time slots of a frame are orthogonal to each other. With this realization, in accordance with the principles of this disclosure, the inputs signals of each frame are mapped onto a coordinate system dictated by the symbols of the previous frame, and symbols from a constellation are selected based on the results of such mapping. Received signals are detected by preprocessing the signals detected at each antenna with signals detected by the antenna at the immediately previous frame, and then applied to a maximum likelihood detector circuit, followed by an inverse mapping circuit.